The time of flight (TOF) technique for determining a distance to an object is well known in the art. This TOF technique comprises sending a light signal towards an object and measuring the time taken by the light signal to travel to the object and return back. Generally, the measurement of the time is accomplished by measuring a time shift between the emitted light signal and the detection by a light sensor of a return signal reflected from the object. With knowledge of this time shift t, in consideration of the fixed speed of light c, one can easily calculate the distance d to the object (d=c*t/2).
TOF detectors, and in particular TOF image sensors, are used in a wide range of applications spanning from gaming to imaging to security and to safety. In many of these TOF imaging applications, high frame rate and high spatial resolution are system design requirements. Conventional TOF detectors, however, are unable to satisfy these system design requirements.
FIG. 1 shows a block diagram of a conventional TOF detector. A light emitter 10, typically in the form of an infrared laser emitter (such as a vertical-cavity surface-emitting laser (VCSEL) diode), periodically emits a light signal 12 toward an object 14. The photons reflected from the object 14 produce a reflected (return) light signal 16 directed back toward a photosensor 18. The photosensor 18 is typically a single photon avalanche diode (SPAD). In a preferred implementation, multiple SPADs are provided in a detector array 20. The SPAD photosensors 18 of the SPAD array 20 are configured in avalanche mode and in response to receiving a photon of the reflected light signal 16, the SPAD photosensor 18 produces a detection (avalanche) pulse 22. Because the SPAD array 20 includes many SPAD photosensors 18, a plurality of detection pulses 24 are generated for each light signal 12 emission. These pulses, collectively referred to as a return pulse 25 (see, FIG. 2), will occur over a range of time due to a number of well-known factors. A measurement circuit 26 is provided to determine a midpoint 28′ of the return pulse 25 (i.e., where one-half of the detection pulses 22 of the return pulse 25 occur before the midpoint and one-half of the detection pulses occur after the midpoint). The time difference between a reference signal 28 indicating the timing of the emitted light signal 12 and the detected midpoint 28′ represents the time of flight and is then used to calculate the distance to the object.
It is known in the art to implement the measurement circuit 26 as an analog circuit (see, United States Patent Application Publication No. 2013/0077082, incorporated herein by reference) and as a digital circuit (see, United States Patent Application Publication No. 2016/0047904, incorporated herein by reference). Improvements in the design and operation of the measurement circuit 26 are, however, needed in order to support a desired increase in frame rate and spatial resolution.